1. Field of Invention
The present invention relates to compositions comprising oxime silane end-capped polymers. More particularly, the invention relates to curable silicon compositions, such as room temperature vulcanizable (RTV) having a oxime cross-linking agent, and to oxime silane end-capped organic polymers, such as polyurethane.
2. Description of Related Art
A silicone sealant or adhesive is a powerful, flexible product that can be used in many different applications. Silicone sealant remains very flexible, even once it has fully cured. Silicone sealant can also withstand very high temperatures, making it ideal for applications involving high heat exposure (e.g., engine gasket sealant). Silicone sealants come in a variety of different forms. For example, one is in a two-component system, requiring the user to mix the two components to make the sealant. The mixture must be made immediately before using it, which makes this type of silicone sealant rather tricky to use.
Silicone sealant may also come as a single product, with no mixing required. One type of single-component silicone sealant is called Room Temperature Vulcanizing (RTV). This form of sealant starts to cure as soon as it is exposed to the air—or, more precisely, the moisture in the air. This simplicity is particularly advantageous in applications such as sealing gaps, joints, and crevices in residential or commercial buildings.
The vulcanizing agent in RTV silicone rubbers is a cross-linking compound or catalyst. In neutral cure silicone sealants, one of the most commercially successful and desired cross-linking systems involves oxime silanes as cross-linker. Silicone sealant and coating systems using these cross-linkers typically enjoy superior shelf life, adhesion, and mechanical properties over that of competing technologies such as acetoxy, alkoxy or amine cross-linked systems. The most abundant and economical of the oxime silanes are those which are characterized by the hydrolyzable groups as being 2-butanone oxime, i.e. methyl ethyl ketoxime (MEKO). For example, U.S. Pat. No. 3,189,576 (Sweet) describes oxime silanes having the general formula (X═NO)aSiR′4-a, methyl ethyl ketoxime.
Despite their popularity, RTV silicones utilizing MEKO as a cross-linking agent have a number of shortcomings. For example, they often suffer from a lack of clarity. (See, e.g., EP660838 B1 describing how the typical use of oxime silane cross-linkers containing methyl ethyl ketoxime provide opaque or cloudy rubber when reacted hydroxyl terminated polydimethylsiloxanes.) Silicone rubber compositions which cure to a clear state are highly desired in the commercial marketplace. Also, an MEKO-based RTV silicone generates MEKO as a volatile by-product via a hydrolysis reaction as the silicone cures. Due to concerns regarding the possible carcinogenic effect of MEKO, in some markets, it is desired to limit the amount of MEKO in RTVs such that the uncured sealant contains less than 1 wt. % of free MEKO.
To limit the amount of MEKO in a sealant, it has been suggested to substitute this compound with various other cross-linking agents. However, all of the substitutes heretofore known in the art have significant disadvantages. For example, one common method of limiting the amount of MEKO in a sealant is to employ silane hybrids which contain both 2-butanone groups and alkoxy groups on the silane cross-linker. However, it is well known that such sealants using these hybrids have insufficient shelf life, require the use of special catalysts and mandate undesirable mix procedures. In addition, such hybrids typically have less than desirable rheological properties.
Other solutions to this problem include the use of silanes containing acetone oxime (propanone oxime). However, this oxime has been found to cause tumors in males rats which strongly suggests that the compound has a toxicity similar to 2-butanone oxime. In addition, since 2-propanone oxime sublimates at fairly low temperatures, sealants which employ this type of cross-linker, when stored in tubes which experience warm temperatures, will find that the sealant bubbles and undesirably yellows during oven aging.
EP660838, suggests using methyl isobutylketoxime (MIBKO) in combination with MEKO to reduce the MEKO concentration in RTV sealants. However, MIBKO-based RTVs have a prolonged period of outgassing of residual oxime which has an unpleasant odor. These same cross-linkers can also be used in combination with 2-butanone based oxime silane cross-linked systems such that the level of free 2-butanone oxime released by the sealant is reduced. However, such sealants still are deficient in that they are marked by a prolonged period of residual odors. Such disadvantages are especially relevant in indoor applications where the applied sealant or coating has little or poor ventilation and the residual odors are detected for several days to weeks after the sealant has been applied to cure.
Therefore, currently there exists a substantial need for cross-linking agents that can be used in a RTV silicone product to improve its odor characteristics, including a shortened period of time that residual odors are emitted. Such cross-linking agents can be used alone, or optionally mixed with 2-butanone oxime silane cross-linkers, in a RTV silicone sealant. The present invention satisfies this need among others.